Method and apparatus for electrical heating of oil-bearing formations



p 1964 J. ORKISZEWSKI ETAL 3,

METHOD AND APPARATUS FOR ELECTRICAL HEATING *OF OIL-BEARING FORMATIONS Filed May 4, 1962 2 Sheets-Sheet 1 JOSEPH ORKISZEWSKI 25 JAMES L. HILL FIG. 1 PRESTON s. MQREYNOLDS THOMAS c. BOBERG INVENTORS A TORNEY Sept. 22, 1964 J. ORKISZEWSKI ETAL 3,149,672

METHOD AND APPARATUS FOR ELECTRICAL HEATING 0F OIL-BEARING FORMATIONS Flled May 4, 1962 2 Sheets$heet 2 FIG. 2

JOSEPH ORKISZEWSKI JAMES L. HILL PRESTON S. MQREYNOLDS THOMAS C. BOBERG INVENTORS ATT RNEY United States Patent Office angers Patented E'aept. 22, 1964 3,149,672 METHGD AND APPARATUS FGR ELECTRKIAL I-lEATlNG F QTL-EEARTNG FQRMA'HQNS Jioseph Orlriszewshi, Tulsa, Gilda, James L. Hill, Etiennpaign, Hill, and Yreston S. McReynoids and Thomas C.

Bo erg, Tulsa, Okla, assignors to Jersey Production Research Company, a corporation of Delaware Filed May 4, 1%2, Ser. No. 192,565 '7 Claims. (Cl. 166-39) This invention relates to the production of relatively viscous oil from a subterranean reservoir penetrated by a well bore. A method and apparatus for thermally stimulating the production of oil from such a reservoir by downhole electric heating is provided. More specifically the method of the invention includes the step of passing an electric current between two vertically spaced fractures propped with particles of an electrical conductor. The apparatus includes a downhole transformer and two connection collar assemblies for establishing electrical contact between the propping agent of the fractures and the secondary windings of the downhole transformer.

The method of the invention is primarily a process for thermally stimulating oil production. That is, heat is introduced into the oil-bearing formation for the purpose of reducing the viscosity of the oil, thereby facilitating flow of the oil into the wellbore and thence to the earths surface, either from innate pressure or by pumping. In a broad sense it has been recognized in the past that heat facilitates the production of viscous oil. Various downhole heaters have been developed to provide thermal stimulation. However, such methods have been found unsatisfactory because of the extremely slow rate of heat transfer outward from the wellbore. The present invention overcomes this difficulty by supplying heat directly to a relatively large volume of the oil-bearing formation surrounding a wellbore.

The invention is applicable not only in solving the problem of viscous oil production, but also in the removal of parafiin wax deposits and other petroleum residues from the pores of the formation surrounding the wellbore. Such deposits are frequently troublesome, even in reservoirs where oil viscosity is not great enough to warrant application of the invention.

Broadly the method comprises the steps of fracturing the oil-bearing formation in an upper region thereof, preferably at or near its upper boundary, fracturing said formation in a lower region thereof, preferably at or near its lower boundary, propping these fractures with particles of an electrical conductor, passing an electirc current through the oil-bearing formation between the fractures to heat the formation, and withdrawing oil from the well at a stimulated rate.

A more limited embodiment of the method includes the steps of forming a substantially horizontal, metallic propped fracture at or near the upper boundary of the oil-bearing formation, and forming a second substantially horizontal, metallic propped fracture at or near the lower boundary of the oil-bearing formation. The fractures are extended radially a distance of 5-500 feet from the wellbore. Electrical contact is established between the fractures, which serve as electrodes, and a surface source of electric power. The voltage drop between th fractures is increased until the power input ranges from 10 watts to 10 kilowatts, per foot of sand thickness lying between the fractures.

An alternating current source is preferred, since direct current would cause a rapid deterioration of the propping agent, due to electrolysis. Moreover, the use of alternating current permits the convenient use of a downhole transformer to reduce line losses in transmitting power downhole.

The well completion of the invention comprises a first metallic propped fracture extending radially from the wellbore in an upper portion of the oil-bearing formation, and a second metallic propped fracture extending radially from said wellbore in a lower portion of the formation. The completion includes regular steel casing to within two hundred feet of the producing oil sand and non-conductivecasing from this depth through the sand. The metallic propped fractures preferably extend from notched intervals in the borehole wall, where the casing is substantially cut away, as opposed to ordinary perforations in the casing. Next, the non-conductive casing is perforated in the producing interval. The tubing string which extends opposite the producing formation is equipped with a downhole electrical transformer, connection collars, and rubber cups or packers to prevent short-circuiting by borehole fluids.

It will be readily appreciated that preferred operation includes the use of fractures which are substantially horizontal, or more specifically, fractures which are substantially parallel to the boundaries of the oil-bearing formation, in the event that the boundaries are not horizontal. However, present fracturing techniques do not always ensure uniform horizontal fracture propagation. Irregular fractures do not pose a severe problem for purposes of the invention, however, especially in thick formations where the fractures are vertically spaced apart a considerable distance. Of course, the upper and lower fractures must not be allowed to interconnect at any point, since the resulting short circuit would defeat the basic purpose of the invention. The fractures are vertically spaced a distance of about three feet up to as much as one thousand feet or more, depending upon the thickness of the oil-bearing formation. Usually the spacing is from 10 feet to feet.

The successful operation of the invention depends upon the ability of the oil-bearing reservoir to conduct an electric current. It is Well known, however, that oil sands do inherently possess substantial conductivity, due to the presence of at least small amounts of saline connate water. The resistivity of oil-bearing, unconsolidated sands usually ranges from about one to about fifty ohm-meters. Consolidated oil-bearing sandstones are generally less conductive, having a resistivity in the range of about ten to about one thousand ohm-meters.

FIGURE 1 shows a cross-sectional view of the oil producing formation and the borehole, including the complete assembly of the invention. v

FIGURE 2 is a top view of the connection collar assembly which is mounted on the tubing string opposite each fracture.

FIGURE 3 shows a cross-sectional view of the collar assembly taken along the line A-A of FIGURE 2.

Referring now to FIGURE 1 in detail, a cross-sectional view of the earth is shown which includes oil-bearing formation l1, overburden 12, and underburden 13 penetrated by wellbore 14. The well completion of the invention includes ordinary steel casing 15, plastic or other non-conducting casing 16 having perforations 17 therein, and larger openings or notches l8 and 19 near th upper and lower boundaries, respectively, of the oil-bearing formation. Extending radially from the wellbore into the formations at the level of notched interval 18 is a metallic propped fracture 20. A similar fracture 21 extends radially from the wellbore at the level of notched interval 19.

Suspended within wellbore l4 and mounted on tubing string 22 is an assembly of elements comprising downhole transformer 23, connection collars 2d and 25, input cable 2s, cable 27 which connects collar 24 with the secondary winding of transformer 23, and cable 28 which connects collar with the secondary winding of the transformer. Bristles 2% extend in contact with the metallic propping agent of fracture 2t) completing the electrical connection with the secondary winding of transformer 23. Similarly, the bristles of collar 25 establish contact with the propping agent of fracture 21, completing th electrical connection between the propping agent and the secondary winding of the transformer.

The downhole transformer is employed in order to permit the transmission down the well of a high voltage, low current power supply which is then transformed downhole into a low voltage, high current power supply thereby reducing line losses in transmitting power down the well.

The purpose of the connection collar assemblies is to establish electrical contact between the metal propping agent of the fractures and the secondary winding of the transformer. The assembly is preferably a radial metallic brush with the bristles connected to the secondary winding and insulated from the tubing string. As the tubing string is lowered into the well the bristles are bent up along the casing wall, and as the assembly passes the notched intervals in the borehole wall opposite the fractures, the bristles spring out, making contact with the propping agent. Slight motions of raising, lowering and rotating the tubing string are sometimes necessary to establish firm contact between the collar bristles and the propping agent in the fractures.

Opening 37. is provided within tubing string 22 at a point beneath transformer 23 v hereby crude oil produced from the formation flows through the center of transformer 23 and serves as a cooling medium for said transformer.

Conventional packers 32 and 33 are provided in order to seal the borehole and thereby prevent the collection of fluids within the borehole to the extent which would cause short-circuitin g between the respective collar bristles. Packer 34 is also desirable, to prevent formation fluids from entering the wellbore through notched interval 18, and to prevnt wellbore fluids from entering the formation.

For the purpose of carrying out the invention, plastic or other non-conducting casing 16 must extend throughout the producing interval. The reason for this is that ordinary steel casing would cause short-circuiting directly between the connection collars and thus prevent any substantial penetration of the electric current into the formation to stimulate the production of oil.

Ordinary perforations 17 are provided throughout the major portion of the producing interval in order to accommodate the influx of oil. Openings 1% and 1% pro vided near the upper and lower extremities of the producing interval are much larger than perforations 17 in order that they may accommodate bristles 29 and 3d of the connection collars. Propped fractures Ztl and 21 are then provided by any conventional techniques, an example of which is disclosed in US. 2,802,531, the details of which are not essential to a complete disclosure of the present invention. The metallic particles may be aluminum spheres, for example. Other metals are also suitable, including iron, copper, magnesium, and zinc. Carbon particles are also suitable.

Next, the downhole transformer 23, connection collars 29 and 3t and packers 32, 33 and 34 are mounted on tubing string 22, lowered into the wellbore, and positioned opposite the producing interval such that bristles 29 and 39 can be made to establish contact with the propping agent within fractures 20 and 21.

The coils of transformer 23 are wrapped on a hollow core which surrounds tubing string 22. As an example, the transformer may operate from a 4,160 volt supply source with an approximate primary to secondary turns ratio of 500 to 1 and a power rating of 15 to 30 kilowatts. Preferably, the transformer utilizes the produced oil as a cooling fluid and thus operates at temperatures below 250 F. Such a transformer has a minimum efiiciency of 90 percent.

Referring now to FIGURE 2, a top view of connection collar 24 is shown which includes bristles 29 extending radially therefrom. The collar is mounted on tubing string 22 with a separation of the collar from the tubing by a band of electrical insulation 41. An opening 42 is provided through which the input cable 26 extends. Bristles 29 are made from a highly conductive, resilient material. As an example each bristle may be a steel strip one inch by one-tenth of an inch and about 12 inches long.

Collar 25 and bristles 30 are essentially identical to collar 24 and bristles 29, the only difference being that no opening 42. is required therein. Accordingly, collar 25 is not separately illustrated or described in detail.

Referring now to FIGURE 3 bristles 29 are connected to collar 24 by means of bolts 44. Collar 24 is in turn connected to tubing 22 by means of bolts 43. Bolts 43 must be non-conducting in order to maintain the electrical insulating of collar 24 from tubing 22. Cable 27 from the secondary winding of transformer 23 is connected to collar 24 by bolts 44.

The power requirement for the process ranges from about 10 watts to about 10 kilowatts, per foot of producing sand thickness. The preferred range is from about watts to about 1.0 kilowatt, per foot of producing sand thickness, depending on the degree of heating desired in a given formation.

At equilibrium conditions, the temperature rise within the heated zone of the reservoir depends primarily upon the power input per foot of sand thickness, and the rate of oil flow, which acts to remove heat from the Zone. Within the preferred range of power input, as stated above, the temperature of the heated zone will be raised at least about 25 F., where the initial rate of drainage is 0.80 barrel per day, per foot of sand thickness, and as much as 890 P. where the initial production of oil is is only 0.25 barrel per day, per foot of sand thickness.

As a specific example of the invention, calculations show that a stimulated flow rate of 100 barrels of oil per day is attained from an expenditure of 17.5 kilowatts of efiective power input to a system having the following characteristics:

Well depth ft 2800 Producing sand thickness ft 50 Drainage radius ft 500 Well base radius ft 0.25 Fracture radius ft 60 Volumetric heat capacity, F. B.t.u./ft. 31.25 Initial reservoir temp. F 84 Initial oil viscosity cp 432 Unstimulated flow rate bbls./day 50 What is claimed is:

1. A method for thermally stimulating the production of oil from a subterranean reservoir penetrated by a wellbore, which comprises fracturing the oil-bearing formation in an upper region thereof, fracturing said formation in a lower region thereof, propping said fractures with particles of an electrical conductor, connecting said fractures with a source of electric current, passing an electric current between said fractures to heat said formation, and withdrawing oil from said Wellbore at a stimulated rate.

2. A method as defined by claim 1 wherein said fractures are substantially horizontal.

3. Apparatus for electrically heating a subterranean oil-bearing formation penetrated by a wellbore, said formation having two vertically spaced conductor-propped fractures extending radially from said wellbore, which comprises a transformer, an electrical input cable connected to the respective terminals of the primary Winding of said transformer, a connection collar assembly at each end of said transformer electrically connected to the respective terminals of the secondary winding of said transformer, said transformer and said collar assemblies being adapted to be mounted on a tubing string and lowered therewith into said wellbore to a position opposite said oil-bearing formation, said collar assemblies being spaced from each other along said tubing string a distance substantially equal to the vertical spacing of said fractures, and said collar assemblies comprising means for establishing electrical contact with the propping material in said fractures.

4. Apparatus as defined by claim 3, wherein said fractures are substantially horizontal.

5. A method for thermally stimulating the production of oil from a subterranean reservoir penetrated by a wellbore, which comprises forming a substantially horizontal, metallic propped fracture near the upper boundary of the oil-bearing formation, forming a substantially horizontal, metallic propped fracture near the lower boundary of said formation, establishing electrical contact between said fractures and a surface source of electric power, increasing the voltage applied to said fractures until the power input to the formation falls within the range of 100 watts to 1.0 kilowatts, per foot of oil-bearing formation, and producing oil from said formation at a stimulated rate.

6. A method for thermally stimulating the production of oil from a subterranean reservoir penetrated by a wellbore, which comprises forming a metallic propped fracture near the upper boundary of the oil-bearing formation, forming a metallic propped fracture near the lower boundary of said formation, establishing electrical contact between said fractures and a surface source of electric power, establishing a voltage drop between said fractures which corresponds to a power input to the formation within the range of 10 watts to 10 kilowatts per foot of oil-bearing formation, and producing oil from said formation at a stimulated rate.

7. A method as defined by claim 6, wherein said fractures are substantially horizontal.

References Cited in the file of this patent UNITED STATES PATENTS 849,524 Baker Apr. 9, 1907 2,634,961 Ljungstrom Apr. 14, 1953 2,795,279 Sarapuu June 11, 1957 2,801,090 Hoyer et al. July 30, 1957 

1. A METHOD FOR THERMALLY STIMULATING THE PRODUCTION OF OIL FROM A SUBTERRANEAN RESERVOIR PENETRATED BY A WELLBORE, WHICH COMPRISES FRACTURING THE OIL-BEARING FORMATION IN AN UPPER REGION THEREOF, FRACTURING SAID FORMATION IN A LOWER REGION THEREOF, PROPPING SAID FRACTURES WITH PAR-F TICLES OF AN ELECTRICAL CONDUCTOR, CONNECTING SAID FRACTURES WITH A SOURCE OF ELECTRIC CURRENT, PASSING AN ELECTRIC CURRENT BETWEEN SAID FRACTURES TO HEAT SAID FORMATION, AND WITHDRAWING OIL FROM SAID WELLBORE AT A STIMULATED RATE.
 3. APPARATUS FOR ELECTRICALLY HEATING A SUBTERRANEAN OIL-BEARING FORMATION PENETRATED BY A WELLBORE, SAID FORMATION HAVING TWO VERTICALLY SPACED CONDUCTOR-PROPPED FRACTURES EXTENDING RADIALLY FROM SAID WELLBORE, WHICH COMPRISES A TRANSFORMER, AN ELECTRICAL INPUT CABLE CONNECTED TO THE RESPECTIVE TERMINALS OF THE PRIMARY WINDING OF SAID TRANSFORMER, A CONNECTION COLLAR ASSEMBLY AT EACH END OF SAID TRANSFORMER ELECTRICALLY CONNECTED TO THE RESPECTIVE TERMINALS OF THE SECONDARY WINDING OF SAID TRANSFORMER, SAID TRANSFORMER AND SAID COLLAR ASSEMBLIES BEING ADAPTED TO BE MOUNTED ON A TUBING STRING AND LOWERED THEREWITH INTO SAID WELLBORE TO A POSITION OPPOSITE SAID OIL-BEARING FORMATION, SAID COLLAR ASSEMBLIES BEING SPACED FROM EACH OTHER ALONG SAID TUBING STRING A DISTANCE SUBSTANTIALLY EQUAL TO THE VERTICAL SPACING OF SAID FRACTURES, AND SAID COLLAR ASSEMBLIES COMPRISING MEANS FOR ESTABLISHING ELECTRICAL CONTACT WITH THE PROPPING MATERIAL IN SAID FRACTURES. 